Enhancers are DNA sequences which play an important role in the transcription of proteins. A number of different enhancer sequences have been identified from different sources, including enhancers of vital origin, e.g. from the SV40 virus, and of cellular origin. As regards the immunoglobulin (Ig) genes, enhancer elements have been identified in the major introns of both the heavy chain (IgH) (Banerji et al., 1983; Gillies et al., 1983; Neuberger, 1983) and kappa (identified herein as "K") light chain (Picard and Schaffner, 1984; Queen and Stafford, 1984) loci.
The present invention is based on the identification of a novel enhancer in the mouse immunoglobulin K locus, downstream of C.sub.K, in addition to the K-intron enhancer. Functionally equivalent enhancers are to be expected to occur downstrem of C.sub.K of the immunoglobin K locus of other species.